Whether you are building your new ADU for family or to rent it out , the income that you may derive as a rental is well worth the cost . Garage conversions are getting $1600 to $2200 per month . If you obtain separate meters you can have tenants pay the utilities . Larger detached ADU's can rent out for $2500 to $4,000 depending on the size of the ADU and the location of your property . A 1200 sqft ADU is equivalent to a 1200 sqft house without the yardwork . Best of all the property value will increase according to the size of the ADU and the rent it receives. Compared to a duplex which is usually in a neighborhood of rental properties , over crowded with parking and not kept up versus a neighborhood with owner occupied properties , your property will be more attractive to potential renters . If you plan on selling Real estate agents are finding 70% of buyers are looking to buy a house with an ADU already built on it , in researching the additional value of an ADU to the property the value added will be square footage as your existing house is $450 to $650 a square foot in California , a new 800 sqft ADU can be worth $400 to $500 k more to the value of your house . The cost to build an ADU is $250 to $350 a square foot .